Process For Obtaining Protective Coatings Against High Temperature Oxidation

ABSTRACT

The invention relates to a process which allows obtaining protective coatings against high temperature oxidation based on MCrAlY in which M is selected from the group formed by Ni, Co or Fe or their alloys, and comprises the thermal spray of MCrAlY-based powders by high frequency pulse detonation (HFPD) techniques. A high density ceramic layer is optionally deposited on the MCrAlY layer by high frequency pulse detonation (HFPD) techniques.

OBJECT OF THE INVENTION

The present invention relates to a process for depositing MCrAlY-based powders on a substrate to obtain a protective coating against high temperature corrosion-oxidation.

The process of the invention allows obtaining quality protective layers, with a high productivity and a low cost.

It is also an object of the invention that the MCrAlY layer obtained is useful as an anchor for a thermal barrier.

BACKGROUND OF THE INVENTION

MCrAlY-based coatings, in which M is selected from Ni, Co and Fe, are usually used for the protection of metal components in high temperature environments, such as for example the blades or casings of a gas turbine.

The purpose of these coatings is to protect the metal substrate against high temperature corrosion and oxidation, for which reason a layer formed by a ceramic thermal insulator or thermal barrier is applied on them.

These coatings are deposited by means of different thermal spray techniques, and especially by means of vacuum plasma spray (VPS) techniques, air plasma spray (APS) technique, HVOF techniques or detonation processes.

The validity of these coatings is directly related to their density and internal cohesion and generally to a microstructure preventing the presence of pores and cracks allowing the corrosive attack on the substrate.

The chemical composition of MCrAlY and especially the presence of aluminium and yttrium, which in the conditions of use act by causing the formation of a well-adhered protective aluminium layer, are also very important.

In this sense, the generation of oxides during the spray deposition reduces the available amount of aluminium and yttrium and therefore the protective behavior of the coating in use.

Vacuum plasma spray (VPS) techniques produce high quality MCrAlY coatings and high thermal performance because they are coatings with a high density and without the presence of oxides as a result of the fact that they are made in a closed chamber with a controlled atmosphere. However, they have the drawback that they are expensive and have a low productivity, as well as the dimensional limitations for the parts to be treated derived from the need to use vacuum chambers.

Due to these drawbacks, vacuum plasma spray techniques are not generally used in the industry to obtain this type of protective coatings.

For this reason, alternative thermal spray techniques in atmospheric conditions are used, such as APS or HVOF techniques for example. Detonation techniques, known as D-Gun, according to U.S. Pat. No. 2,714,563 are also used but none of them has been successful due to their low density in the case of APS techniques or the presence of oxides in the coating in the HVOF or D-Gun techniques for example.

HVOF and detonation (D-Gun) techniques produce a high velocity gas flow as a result of a continuous combustion or pulse explosion process which can accelerate the spray particles and obtain very dense coatings. However, it is extremely difficult to control the oxidation of the obtained coatings given the oxidizing nature of the fuel mixtures that are necessary to obtain high velocity flows.

U.S. Pat. No. 5,741,556 describes a process for producing MCrAlY coatings by means of detonation (D-Gun) processes, with an oxide dispersion in the coating suggesting the modification of the coating powders, increasing the initial presence of aluminium in the powders to compensate for the loss thereof due to the formation of oxides in the spray process.

U.S. Pat. No. 6,366,134 describes a family of MCrAlY powders specifically designed for HVOF spraying with increased yttrium levels to compensate for the oxygen-rich atmosphere of the HVOF process and the subsequent formation of yttrium oxides, the generation of which would reduce the availability of yttrium necessary for the improvement of the durability and properties of the protective oxides generated by MCrAlY layers in conditions of use.

The drawback of the processes described in these two patent documents is that they introduce modifications in the chemical equilibrium of the different coating elements, it being difficult to control the degree of oxidation generated during the spray process, which finally affects the features of the coatings.

On the other hand, when MCrAlY coatings are used as an anchoring layer for a ceramic coating acting as a thermal barrier coating (TBC), the aluminium oxides formed in the MCrAlY during the spray process can cause the ceramic layer to peel off or to come off.

To solve this problem, a second MCrAlY coating with a greater capability to form alumina is applied on the MCrAlY so as to improve the adhesion and compatibility of the thermal barrier to be applied on this second layer. A process of this type is described for example in European patent no. 1 327 702. However, this compositional modification of MCrAlY in this surface layer can affect its protective capability against a corrosive medium at high temperatures, and in any case it requires a specific deposition, increasing the complexity of the process.

Treatments are sometimes carried out on MCrAlY coatings or MCrAlY layers that are multi-stratum or have a gradual composition the outer layer of which has a good adhesion for the thermal barrier are used. This type of coatings are described in the following patents for example: U.S. Pat. No. 5,894,053, DE19842417 and U.S. Pat. No. 5,942,337. However, any of these processes introduces an additional complexity to the process, increasing the costs and the application difficulties.

In any case, there is currently no known process which allows simultaneously obtaining MCrAlY coatings with high productivity, high quality and a reduced price and on which a ceramic layer acting as a thermal barrier against high temperatures can in turn be applied with good adhesion features.

DESCRIPTION OF THE INVENTION

The process object of the invention allows obtaining a coating against high temperature corrosion and oxidation based on the thermal spray of commercial MCrAlY powders using high frequency pulse detonation (HFPD) techniques which allow obtaining a high density, low oxidation coating with a high productivity and low cost.

Furthermore, when the MCrAlY is to be used as an anchoring layer for a thermal barrier, a ceramic layer can be sprayed on it using the same HFPD technique, thus achieving a well-adhered, very dense thin layer which prepares the MCrAlY with a ceramic outer surface having good compatibility with thermal barriers. Said porous thermal barriers can be deposited using any thermal spray technique.

High frequency pulse detonation (HFPD) spray techniques are described in the following applications for example: WO97/23299, WO97/23301, WO97/23302, WO97/23303, WO98/29191, WO99/12653, WO99/37406 and WO01/30506.

These techniques use the gas flows produced during the cyclic explosions or detonations to accelerate and spray the coating material and differ from the detonation techniques known as D-Gun in the absence of mechanical valves or other mobile elements, the pulsed behavior being achieved by the dynamics of the detonation process, from a continuous gas supply.

Electronically controllable high frequency explosions which can exceed 100 Hz compared to the frequencies of a D-Gun process working between 1 and 10 Hz, are thus achieved.

This process allows generating explosions with a high temperature range using combustion gases such as methane and natural gas or propane, propylene, ethylene or acetylene type gases, using oxygen-rich mixtures and controlling the amount of gases involved in each explosion.

This technique allows the deposition of all types of materials, from metallic to ceramic alloys achieving a good adherence and compaction as a result of the detonation process.

The deposition of MCrAlY powders by means of the aforementioned high frequency pulse detonation technique requires optimizing the process parameters which allow obtaining a high density, good compaction and adherence of the coating with minimal internal oxidation, thus requiring a low temperature of the detonation process and a low oxygen environment during the spray.

Gases generating low temperature combustion such as methane or natural gas mixed with a dilution of inert gases such as nitrogen, argon, helium or others, are specifically used, using oxygen as a combustion agent in order to achieve low oxygen-carbon ratios.

Detonation frequencies exceeding 60 Hz are generally used to improve the productivity of the process and optimize the volume of gases used in each explosion.

The MCrAlY powders are introduced in the barrel of the detonation gun in a point close to its exit, at a distance from the detonation chamber between 100 and 500 mm so as to reduce their residence time in the gaseous medium of the spray.

Generally, the MCrAlY coating obtained is later subjected to a heat treatment in a controlled vacuum environment to promote the diffusion process causing a suitable microstructure for protecting against corrosion-oxidation.

When the MCrAlY coating is to be used as an anchoring layer for a thermal barrier, a ceramic layer with a high density and small thickness improving the adherence of the porous thermal barrier on the MCrAlY is also sprayed on it by means of de high frequency pulse detonation (HFPD) techniques.

This ceramic layer can be formed by Al₂O₃, ZrO2-Y2O3 and mixtures of these elements, which can be applied as single layers, multiple layers or gradual composition layers.

High detonation temperatures and oxygen-rich spray environments are required for the spray of this dense ceramic layer with a small thickness acting as an anchor for the thermal barrier, in order to cause the complete fusion of the ceramic particles.

High temperature combustion gases such as propane, propylene, ethylene or acetylene with high concentrations of oxygen as a combustion agent are specifically used in order to achieve a high temperature detonation and highly oxidizing environments allowing the fusion of the ceramic powders.

The frequency of the explosions can be greater than 40 Hz and the ceramic powders are introduced in a point of the barrel close to the combustion chamber to force them to traverse the entire length of the barrel, thus increasing the residence time and favoring heat transfer from the gaseous mixture to the ceramic powder.

The porous thermal barrier can be deposited on the obtained coating by using any thermal spray technique such as VPS, APS or HVOF, for example, or even other techniques, such as PVD for example.

DESCRIPTION OF THE DRAWINGS

To complement the description which is being made and with the aim of aiding to better understand the features of the invention, a set of drawings is attached as an integral part of said description, in which the following has been represented with an illustrative and non-limiting character:

FIG. 1 shows a microstructure with a MCrAlY coating according to the process object of the invention.

FIG. 2 shows a microstructure of a MCrAlY coating and a dense ceramic layer with a small thickness on the latter obtained according to the process object of the invention.

PREFERRED EMBODIMENT OF THE INVENTION

Two examples of MCrAlY coatings obtained according to the process of the invention are described below.

Example 1 MCrAlY Coating

CoNiCrAlY (Amdry 9954) were used as powders for obtaining the coating. The spray was carried out by means of high frequency pulse detonation techniques with the following parameters:

-   -   Natural gas flow (slpm): 59     -   Nitrogen flow (slpm): 62     -   Oxygen flow (slpm): 82     -   Frequency (Hz): 60     -   Nitrogen carrier gas (slpm): 80     -   Spray distance (mm): 150

A coating was obtained with these parameters, and the microstructure of such coating after a high temperature heat treatment can be seen in FIG. 1.

Example 2 MCrAlY+Ceramic Coating

CoNiCrAlY (Amdry 9954) was used as a spray powder to obtain the lower MCrAlY layer. The spray was carried out by means of high frequency pulse detonation (HFPD) techniques with the following parameters:

-   -   Natural gas flow (slpm): 59     -   Nitrogen flow (slpm): 62     -   Oxygen flow (slpm): 82     -   Frequency (Hz): 60     -   Nitrogen carrier gas (slpm): 80     -   Spray distance (mm): 150

Al2O3 (Metco 105SFP) was used as a spray powder to obtain the upper ceramic layer. The spray was carried out by means of high frequency pulse detonation (HFPD) techniques with the following parameters:

-   -   Propylene flow (slpm): 60     -   Oxygen flow (slpm): 180     -   Frequency (Hz): 70     -   Nitrogen carrier gas (slpm): 80     -   Spray distance (mm): 230

A coating was obtained with these parameters, and the microstructure of such coating after a high temperature heat treatment can be seen in FIG. 2. 

1. A process for obtaining protective coatings against high temperature oxidation based on MCrAlY in which M is selected from the group formed by Ni, Co or Fe or their alloys, characterized in that it comprises the thermal spray of MCrAlY-based powders by high frequency pulse detonation (HFPD) techniques.
 2. A process for obtaining protective coatings against high temperature oxidation according to claim 1, characterized in that the thermal spray is carried out using a mixture comprising at least one fuel and one combustion agent.
 3. A process for obtaining protective coatings against high temperature oxidation according to claims 1, characterized in that the thermal spray is carried out using a gaseous mixture formed by methane or natural gas mixed with a dilution of inert gases selected from nitrogen, argon, helium or others and oxygen or air as a combustion agent.
 4. A process for obtaining protective coatings against high temperature oxidation according to claim 1, characterized in that a heat treatment is applied in a controlled atmosphere after the deposition of the MCrAlY layer to cause the diffusion.
 5. A process for obtaining protective coatings against high temperature oxidation according to claim 1, characterized in that a high density ceramic layer is applied on the MCrAlY layer by thermal spray, by means of high frequency pulse detonation (HFPD) techniques, the dense ceramic layer forming an anchor for the application of ceramic material thermal barriers, deposited by means of any thermal spray technique.
 6. A process for obtaining protective coatings against high temperature oxidation according to claim 5, characterized in that the thermal spray is carried out using a mixture comprising at least one fuel and one combustion agent.
 7. A process for obtaining protective coatings against high temperature oxidation according to claim 6, characterized in that the thermal spray is carried out using a mixture of gases selected from propane, propylene, ethylene or acetylene, and oxygen as a combustion agent.
 8. A process for obtaining protective coatings against high temperature oxidation according to claim 5, characterized in that a heat treatment is applied in a controlled atmosphere before or after the deposition of the dense ceramic layer to cause the diffusion. 